Project Summary: Alterations in glutamate neurotransmission are recognized as an important target of pharmacotherapy for alcohol use disorder (AUD). Preliminary investigations with ketamine, a glutamate modulator with potent prefrontal effects, suggest sub-anesthetic infusions may work to facilitate behavioral modification by addressing critical vulnerabilities for a variety of substance use disorders, including AUD. Expanding on a preliminary study suggesting that ketamine reduces number of heavy drinking days (HDD) when combined with motivational enhancement therapy (MET), this 12-week trial powered to detect proportional differences consistent with our prior data (n=120) aims to evaluate whether ketamine promotes a reduction in HDDs relative to an active control (midazolam). We will randomize (1:1) 120 participants seeking treatment for AUD and demonstrating high baseline problem drinking to 2 infusions of ketamine or midazolam separated by 5 weeks (0.71 mg/kg ketamine or 0.025 m/kg midazolam over 52 min). Further, in order to more rigorously assess the impact of behavioral treatment on the efficacy of ketamine, we will employ a two by two factorial (2x2) design, with participants in each medication arm randomized (1:1) either to standard medication management, or to a manualized sequence of motivational enhancement therapy (MET) followed by mindfulness-based relapse prevention (MBRP). MBRP is expected to facilitate relapse prevention after individuals have reduced use or initiated abstinence during MET. We predict that, compared to the control midazolam, ketamine will significantly reduce the proportion of individuals with HDDs. An important secondary hypothesis is that those receiving ketamine and behavioral treatment will demonstrate significantly better outcomes than individuals receiving ketamine alone. Other aims pertain to the effects of ketamine on number of daily drinks, and number of drinking days; the impact of ketamine on time to first HDD or drop-out; and the evaluation of added effectiveness when ketamine is combined with MET/MBRP. If successful, this project stands to contribute significantly to the treatment of AUD, for which new pharmacotherapy strategies are needed. Future studies might test other medications using the design introduced here, as well as focus on clarifying the mechanisms by which ketamine addresses AUD. !